This invention relates to a method and apparatus used to calibrate a measuring instrument. More particularly this invention relates calibration of Scanning Transmission Electron Microscopes (STEM) and Transmission Electron Microscopes (TEM).
Semiconductor manufacturing consists of a number of crucial processing steps performed on wafer lots where measurements of minimum feature sizes known as critical dimensions (CD) are made to ensure proper device fabrication. The high-degree of precision during this processing requires the utilization of Scanning Transmission Electron Microscopes and Transmission Electron Microscopes (S/TEM). These critical tools provide measurement capabilities in the low nanometer range. Accuracy of their measurements is essential since effective process controls depend on CDs they supply. S/TEMs require frequent calibration to ensure their accuracy since processing errors cause appreciable inconsistencies in CDs. Calibration procedures are very time consuming and have a negative impact on semiconductor fabrication workflow. Typical S/TEM magnification calibration approaches are also limited in accuracy.
A conventional calibration approach utilizes a standard (e.g., TEM grid) that possesses one crystal lattice specimen to calibrate a S/TEM for a particular magnification. This approach requires that calibration for each magnification of a range of magnifications required to support semiconductor fabrication use a different standard. Thus, numerous specimen exchanges along with new orientations to achieve calibration over a range of magnifications are required. In addition, the beam conditions must be reset for each standard. Measurements obtained from the specimen""s crystal lattice spacing are compared to known data to determine if the S/TEM requires adjustment or if the magnification results being calibrated are within tolerance. Calibration adjustments are made accordingly. This iterative process is conducted until calibration is achieved for the full range of magnification required in support of semiconductor fabrication.
Another method using a scale with multiple crystal lattice specimens to calibrate a S/TEM is described in U.S. Pat. No. 6,231,668 to Loesch entitled xe2x80x9cMethod for Manufacturing a Calibrated Scale in the Nanometer Range for Technical Devices Used for the High Resolution or Ultrahigh-Resolution Imaging of Structures and Such Scalexe2x80x9d. This patent discloses utilization and manufacturing of a layered scale. Depositions of at least two different crystalline or amorphous materials are used to create the scale. These materials are deposited on a substrate in alternate layers, one on top of the other. Even though a plurality of crystal lattice materials are used, this scale results in a single multi-layered specimen also known as sample on a substrate so the range of calibration of magnification is still limited very limited.
A few other approaches allow calibration over a range of magnifications. However, these standards are limited and do not cover the entire range of S/TEM magnifications typically required to support current semiconductor fabrication requirements. Nor do these solutions provide the accuracy that can be achieved with known crystal lattice plane spacing. It would be very advantageous to utilize a standard that is comprised of a plurality of specimens each having or arranged to present a different lattice spacing or atomic spacing on a single TEM grid. Utilization of this standard would allow rapid accurate calibration of the S/TEM at all desired magnifications and conditions to overcome limitations of prior art. Then, fast thorough calibration can be performed streamlining workflow of a semiconductor processing line.
An object of the present invention is to streamline semiconductor fabrication workflow, solve the need for greater accuracy, and induce process reliability/repeatability by enabling rapid, accurate calibration of high-resolution electron microscopes.
Another object of the present invention is to provide a method and apparatus for rapid high-resolution electron microscopes calibration over the range of magnifications required utilizing a single standard that provides multiple specimens, each possessing different lattice spacing.
In accordance with the present invention, the disclosed method and apparatus enables a calibration process of an electron microscope to be performed where only one standard is required for calibration of the entire range of magnifications. The enabling apparatus consists of different atomic structures such as crystal lattice spacing samples also known as specimens that are resident on the single standard. These samples are arranged in a grid pattern. Lattice spacing dimensions of the samples are known reference data. The S/TEM is adjusted to establish viewing of a single sample from a plurality of samples by bring fringes of the lattice space into focus. Measurements of these lattice spacings are compared to known reference measurement data. If S/TEM measurements do not agree with the lattice spacing dimensions then, the S/TEM magnification is adjusted (i.e., the input current to the lens is adjusted) to reflect known reference data. Sequentially viewing one sample at a time and performing S/TEM adjustments for the particular sample accomplishes calibration. All samples required to support magnification calibration of a range of magnification are used.